High voltage surge arresters are common in the field of electric power transmission and distribution. They may form part of a dissipative system by means of which currents are dissipated upon transient overvoltages occurring in the electric system. Such an arrester may comprise a plurality of varistor blocks, i.e. blocks made of a material that, upon being subjected to a predetermined voltage, turns from a highly resistive state into an electrically conducting state. The arrester may be connected to ground on one hand, and an electric network on the other hand. The varistor blocks need to be electrically insulated and they are insulated as well as protected from the environment by means of an electrically insulating enclosure, thereby forming the surge arrester. Surge arresters of this kind are predominantly located in open air, i.e. in an outdoor atmosphere in which they are subjected to typical outdoor conditions such as deposition of water and dust particles on said enclosure/insulator.
It is a well known fact that such an environment will promote the phenomena of tracking and erosion in the insulator due to leak currents that appear on the surface of the insulator due to deposition of dust and water on said surface. In order to overcome such problems, silicone rubber has been suggested as an outermost electric insulator material, since it has sufficient hydrophobic properties as well as dielectric properties. Silicone rubber also presents mechanical properties that make it suitable as an insulation material.
Apart from the requirements on hydrophobic and dielectric properties, it is also required that the insulator should present sufficient flame retarding properties in the event of an overloading and following short-circuit of the arrester. Therefore, according to prior art, particles of aluminium trihydrate has been mixed into the silicone rubber. Since aluminium trihydrate also has a higher thermal conductivity than the silicone itself, the addition thereof also results in an increase of the thermal conductivity of the silicone rubber. Since the varistor blocks are heated both during normal operation and, in particular, when limiting transient overvoltages, the increase of thermal conductivity obtained as a result of the addition of the aluminium trihydrate is a welcome effect, since it promotes the cooling of the varistor blocks and reduces the risk of overheating thereof. However, the contribution to the increased thermal conductivity of the silicone rubber is still rather restricted, since the thermal conductivity of aluminium trihydrate is not so high. A thermal conductivity above 0.6 W/mK cannot be suspected for this kind of prior art silicone rubber suitable for outdoor insulation applications.
In surge arresters of prior art, the thermal conductivity of the silicone rubber forming the insulator has not been a major problem, since the conductivity conferred by the addition of the aluminium trihydrate and the thermal conductivity of the silicone itself has been sufficient for the transfer of the heat generated by the varistor blocks. However, future surge arresters provided with varistor blocks are conceived to be able of operating with higher electric field strengths than today's surge arresters and will hence emit more heat per unit length than contemporary arresters.
During normal operation only a small current is conducted through the varistor elements but this current increases strongly with both temperature and voltage. Today's arresters are operated continuously at approximately 80% of their rated voltage, this is to some extent due to that the continuous power losses will be too high if the arresters are subjected to a higher continuous operating voltage. If the thermal characteristics of the arresters could be improved also a higher relative continuous operating voltage could be applied which in turn gives a possibility to improve the protection performance of the arrester.
So-called heat sinks (metal blocks of for example aluminium or steel) located between individual varistor blocks are sometimes used in prior art for the purpose of reducing the heat of the varistor blocks. The heat sinks will, however, result in an increase of the length of the surge arrester. This is of disadvantage since it will require the manufacture of longer insulators, which is costly.